Beginning in 1970, Jerry Garcia played low-key weekly gigs with jazz organist Merl Saunders, bassist John Kahn and drummer Bill Vitt when the Grateful Dead weren’t touring. What’s most remarkable about this four-disc set, which compiles two 1973 dates, is the range: They jam on Rodgers and Hart as well as Jimmy Cliff, and turn out the Funkadelic-like instrumental “Keepers”; Garcia also croons an unexpectedly sweet version of “Positively 4th Street” amid soaring, melodic guitar solos. There are no rules – just four luminaries testing their own boundaries.